1. Field of the Invention
This invention pertains to a circuit for converting an interlaced video image, into a pair of de-interlaced video signals of a form suitable for recording upon paired tracks on a recording medium.
2. Description Relative to the Prior Art
A still video recording format has been introduced in which a small, flexible magnetic disk is used to store up to 50 still video pictures. The still pictures originate "live" with an electronic camera or by transfer from another media (e.g. photographic film). The pictures are played back by inserting the recorded disk into a video disk player, which converts the video signals recorded on the disk into a suitable format (e.g., NTSC) television signal. The recording format provides the option of storing a full frame (two circular tracks per picture) for 25 pictures, or a single field (one circular track per picture) for 50 pictures. Which option is preferred depends upon such considerations as storage density, picture quality, and the cost and complexity of the camera used in recording and the player used in playback. Where picture rendition is the paramount consideration, full frame recording and playback is to be preferred.
A full frame still image, like an ordinary television frame, is composed of two fields that are interlaced upon display. Unlike a television frame, however, both fields should be captured concurrently in order to avoid inter-field subject movement, which leads to jitter in a played-back still picture. Simultaneous field capture is provided by a full-frame imager, which collects a charge pattern corresponding to a fully interlaced video frame (see, for example, the line-addressed bucket-brigade structures and the modified CCD frame transfer structures described in "Interlacing in Charge-Coupled Imaging Devices," by C. H. Sequin, IEEE Trans. on Electronic Devices, June 1973, pp. 535-541). The usual full-frame imager provides simultaneous pickup and sequential readout of two fields (see, for example, the solid state image pickup device disclosed in U.S. Pat. No. 4,541,016). Sequential processing, however, poses a problem: Besides being exposed at the same time, both fields should be equally treated during signal processing and then recorded simultaneously. If one field is held on the imager longer than the other, the later-recorded field is likely to accumulate more noise-related artifacts, such as dark current, than the earlier-recorded field.
The still video recording format calls for rotating the magnetic disk at the video field rate (i.e. 3600 r.p.m.). Due to the afore-mentioned noise problem, it is desirable for a pair of magnetic heads to simultaneously record the field signals on adjacent tracks in one revolution of the disk. This requires unloading the interlaced signal from the imager and presenting its de-interlaced counterpart in one disk revolution. Though it is feasible to read out the imager during the field rate (1/60 second) this alone is not enough since the fields remain interlaced. Merely separating the lines by field, as it turns out, is also inadequate since each line remains "time-compressed" relative to the recording (and playback) line standard.